Skeletal System Anatomy, Diagram & Function Written and medically reviewed by the Healthline Editorial Team Co-developed by: In Depth: Skeletal Although a skeleton sometimes symbolizes death and creepy, scary things, it is one of the body’s most life-giving systems. Unlike other living organs, bones are firm and strong, but they have their own blood, lymphatic vessels, and nerves. There are two types of tissue inside bones: Compact bone: This hard and dense tissue makes up the outer layer of most bones and the main shaft of long bones, such as those in the arms and legs. Nerves and blood vessels live inside this tissue. Spongy bone: This tissue is made up of smaller plates filled with red bone marrow. It is found at the ends of long bones, like the head of the femur, and at the center of other bones. Red bone marrow forms most of the blood cells in the body and helps destroy old blood cells. Another type of marrow, yellow bone marrow, resides in the central cavities of long bones. It is mostly made up of fat. However, if the body suffers large amounts of blood loss, it can convert yellow marrow to red to make more blood cells.

The skull consists of 22 separate bones that make up the cranium, the housing for the brain. Twenty-one of those bones are fused together by sutures, nearly rigid fibrous joints. The lower-most bone of the skull is the mandible, or jawbone. The spine, or vertebral column, is a series of irregularly shaped bones in the back that connects to the skull. At birth, humans have 33 or 34 of these bones. But bones fuse as we age, and the result is 26 separate bones in the spines of adults. The rib cage is made up of 12 pairs of bones that encase vital organs in the chest. The bones curve from the back at the vertebral column to the front of the body. The upper seven pairs meet with the sternum, or chest bone. The remaining five pairs are attached to each other via cartilage or do not connect. The muscles of the shoulders and arms include the clavicle (collarbone), scapula (shoulder blade), humerus, radius, ulna, and the bones of the wrist and hand.

The hip bones are three sets of bones—ilia, ischia, and pubes—that fuse together as we grow older. These form the majority of the pelvis at the base of the spine as well as the socket of the hip joint. The sacrum—five fused bones and at the bottom of the spine—and the coccyx, or tailbone, make the rest of the bones in the pelvic region. The head of the femur, the largest and longest bone in the body, creates the other half of the hip joint and extends down to form part of the knee. It begins the bones of the leg. The other bones of the leg include the tibia, fibula, and the bones of the ankle and foot. The most common condition that affects bones is fracture, or when a bone endures such a great impact that it breaks. Other common conditions that affect the skeletal system include: Osteoporosis: This is a disease in which the bones become fragile and prone to fracture. Leukemia: This is a cancer of the white blood cells. Osteopenia, osteitis deformans, and osteomalacia: Similar to osteoporosis, these are other types of bone loss.

Scoliosis, kyphosis, and lordosis: These are abnormalities of the spinal curve. About Healthline BodyMaps BodyMaps is an interactive visual search tool that allows users to explore the human body in 3-D. With easy-to-use navigation, users can search multiple layers of the human anatomy, view systems and organs down to their smallest parts, and understand in detail how the human body works. Using detailed 3-D models of body parts-including muscles, veins, bones, and organs-BodyMaps offers a new way to visualize and manage your health. See how the coronary artery delivers blood to the heart, and learn how plaque build-up on artery walls leads to heart disease. Locate the exact location of a pulled muscle or broken bone, and find information on how to prevent injuries. View a cross-section of the human brain, and learn which areas control certain emotions and body functions. By offering rich, detailed anatomical images alongside links to relevant and useful health information, BodyMaps allows you to leam about your body and your health in a personalized and revolutionary new way. Kids' Health - Topics - Your bones We all have bones. If we didn't, we would be like jellyfish! Bones make up the framework of our bodies.

We call this framework the skeleton. Bones are living, growing and changing parts of our bodies. Babies' skeletons are made up from more than 300 parts, but by the time we become adults we only have 206 bones! No, we haven't lost any! It's just that some of our baby bones are made, partly or completely, of cartilage (say car-til-ej). As we grow bigger and heavier, cartilage is slowly replaced by harder bone. Some smaller bones join together to make one bigger bone. By the time we have finished growing, at about 25 years old, our bones are as big as they are going to get. Most bones have 4 parts: The outside part of bone is called the periosteum (say per-ee-oss-tee-um). This is a thin but very dense layer that has the nerves and blood vessels which nourish the bone. Compact bone is next. It is smooth and very hard. Cancellous (say can-sel-us) bone comes next, and this looks a bit like a sponge but is much stronger. On the very inside of many bones is the bone marrow.

It looks a bit like a jelly and it makes new blood cells for the body. The bones of the skeleton give us our shape and our posture. Without our spine (backbone) and leg bones we would be unable to stand erect. Bones also protect the softer parts of our bodies. The skull is like a natural helmet which protects the brain. The spine protects the nerves in the spinal column. The ribs make a shield around our lungs, heart and liver. Bones are great team players! They act as a base for muscles, ligaments and tendons. Ligaments connect bones to bones, and tendons connect muscles to bones. Our topic 'Your muscles' will tell you more. They all work together to help us move around. Your skull When a baby is born, his or her skull has spaces between its bones. This is to make it easier for the baby to be born. If you look at a new baby you can see that there is a soft spot - called the fontanelle - on the top of her or his head, which is a space between some of the skull bones. As babies grow, the bones of the skull join together and harden. Different bones in the skull have different jobs. Some bones protect the brain.

You can feel the bone at the back of your head and your forehead. Some bones make the framework for your face. You can feel the bones around your eyes. Some bones in the middle part of your ear carry sound from your eardrum to the inner ear. Have a look at the topic Ears – how your ears work for more about these bones. You can feel the jawbone under your chin and where it hinges just under your ear. It is the only bone in your skull that you can move. Your spine The spine has 26 bones which are like circles with wings on the sides and back. They are called the vertebrae (say ver-tu-bray). There are small discs made of cartilage between each vertebra. They act like shock absorbers and also stop the bones rubbing against each other. There is also a hole through each of the vertebra where the spinal cord goes. The top 7 are called the cervical (say ser-vik-al) vertebrae and they support your head and neck. The next 12 are called the thoracic (say thor-ass-ik) vertebrae. These hold one end of your ribs.

The next 5 are the lumbar (say lum-ba) vertebrae. The next, the sacrum (say-krum), is a large bone that is actually made of 5 vertebrae fused (joined) together. At the bottom of the spine is the coccyx (say cok-six), which is one bone made of 4 small vertebrae fused together. (Some people call this the monkey bone or tail bone. Can you work out why?) Your ribs Ribs make a protective cage around your lungs, liver and heart. There are usually 12 pairs of ribs, each pair attached to the spine at the back. Each rib is attached to the ribs above and below it by muscles and ligaments. At the front of the body the top 7 ribs are attached to the sternum (or breast bone). The next 3 pairs are joined to the ribs above, and to the sternum, by cartilage. The last 2 pairs of ribs are called the ‘floating' ribs. But don't panic! They are not attached to the sternum but they are fastened to the spine at the back. If you look at the diagram above, you can see that it's OK!

They are not floating around the body and they don't have sharp edges sticking out! Your arms Each arm is fastened to the scapula (say scap-you-la) - the shoulder blade. The scapula is a triangle shaped bone near the top of the ribs on your back) In the front your arm is fastened to the clavicle (say clav-i-kle) - the collar bone. The arm has 3 bones - the humerus (say hew-mer-us), the radius (ray-dee-us) and the ulna (ul-na). The humerus is the bone between your shoulder and the elbow. That's the one that really hurts when you bang it. It's sometimes called the 'funny bone' because 'humerus' sounds like 'humorous', which is another word for funny or amusing. The radius and ulna bones go from the elbow to the wrist. At the end of the radius and ulna is your wrist. Your wrist has 8 small bones, so it is really flexible. In each hand there are 19 bones! Your hands are so amazing that we have a whole topic about them, so look up Your wonderful hands , and prepare to be amazed!

Your legs are fastened to your pelvis at the hips. The pelvis is shaped like a bowl and supports the contents of your tummy. Legs need to be large and strong to support the rest of the body. The femur (fee-mer) is the largest bone in the body and it goes from the pelvis to the knee. Another bone called the patella ( pat-ell-a), or kneecap, covers the knee joint to protect it. Below the knee there are 2 bones. These are the tibia (tib-ee-a) and the fibula (fib-you-la). They join up to a large bone in the ankle called the talus (tal-uss). There are 6 other bones here so you can move your ankle around but you cannot move it as much as you can move your wrist. Your feet and ankles have a total of 52 bones in them, and as they are very important, they have their own topic too. Look up Look after your feet to find out more. Your joints Where two bones meet is called a joint. Some joints are fixed and don't move - for example, the joints in your skull, before they have grown together.

Some joints move a little, like the joints in your spine. Some joints move a lot. There are two kinds of moving joints. Hinge joints act like the hinge on a door which allows it to open and shut. Some joints are ball and socket joints. These allow movement in all directions. OK, can you work out which joints are hinge joints and which are ball and socket joints? Just move your body around and the answers are easy to find! Look down to the end of this topic to find out if you are right. You only get one body, so you need to make sure that you look after it! Eat healthy food, especially dairy foods, which give your bones the calcium they need. Get plenty of exercise and make sure that you protect your bones when you play sport. Always wear the right sports equipment for the sport you are playing. Wear a helmet when you are on a bike. Wear a helmet, elbow and knee guards when you're skate-boarding, skating or riding a scooter. Interesting 'boney' facts In the olden days, whale bones were used to make corsets - for women and men. These corsets would squash in the person's body so that they looked slimmer or a different shape.

Sounds painful doesn't it? Men who worked on the whaleboats would often be at sea for weeks. Some of them would carve pictures onto whalebones or carve the bone into all sorts of things. This is called scrimshaw (say skrim-shor). Nowadays some of these carvings are very valuable. Animal bones have been used for playing games for thousands of years. Jacks or knucklebones and dice used to be made of bones - nowadays they are usually made of plastic. What boney facts do you know? Maybe you have heard the Gospel song 'Dem dry bones'. It's from the bible story about Ezekial who found himself in a valley full of dry bones and caused them to come alive by God's command. If you want to hear and read the words of this song just type into your search engine “ song, dem dry bones” and you will find lots of different versions there. Some a bit scary but all lots of fun! Join in the dance and move your own bones! If you want to hear and read the song you could go to Were you right? Your knees and elbows are hinge joints and your hips and shoulders are ball and socket joints.

We've provided this information to help you to understand important things about staying healthy and happy. However, if you feel sick or unhappy, it is important to tell your mum or dad, a teacher or another grown-up. Skeletal System – Labeled Diagrams of the Human Skeleton Pectoral (shoulder) girdle Skull The skull is composed of 22 bones that are fused together except for the mandible. These 21 fused bones are separate in children to allow the skull and brain to grow, but fuse to give added strength and protection as an adult. The mandible remains as a movable jaw bone and forms the only movable joint in the skull with the temporal bone . The bones of the superior portion of the skull are known as the cranium and protect the brain from damage. The bones of the inferior and anterior portion of the skull are known as facial bones and support the eyes, nose, and mouth. Hyoid and Auditory Ossicles The hyoid is a small, U-shaped bone found just inferior to the mandible. The hyoid is the only bone in the body that does not form a joint with any other bone—it is a floating bone.

The hyoid’s function is to help hold the trachea open and to form a bony connection for the tongue muscles . The malleus, incus, and stapes—known collectively as the auditory ossicles —are the smallest bones in the body. Found in a small cavity inside of the temporal bone, they serve to transmit and amplify sound from the eardrum to the inner ear. Vertebrae Twenty-six vertebrae form the vertebral column of the human body. They are named by region: Cervical (neck) - 7 vertebrae Sacrum - 1 vertebra Coccyx (tailbone) - 1 vertebra With the exception of the singular sacrum and coccyx, each vertebra is named for the first letter of its region and its position along the superior-inferior axis. For example, the most superior thoracic vertebra is called T1 and the most inferior is called T12. Ribs and Sternum The sternum, or breastbone, is a thin, knife-shaped bone located along the midline of the anterior side of the thoracic region of the skeleton . The sternum connects to the ribs by thin bands of cartilage called the costal cartilage.

There are 12 pairs of ribs that together with the sternum form the ribcage of the thoracic region. The first seven ribs are known as “true ribs” because they connect the thoracic vertebrae directly to the sternum through their own band of costal cartilage. Ribs 8, 9, and 10 all connect to the sternum through cartilage that is connected to the cartilage of the seventh rib, so we consider these to be “false ribs.” Ribs 11 and 12 are also false ribs, but are also considered to be “floating ribs” because they do not have any cartilage attachment to the sternum at all. Pectoral Girdle and Upper Limb The pectoral girdle connects the upper limb (arm) bones to the axial skeleton and consists of the left and right clavicles and left and right scapulae. The humerus is the bone of the upper arm. It forms the ball and socket joint of the shoulder with the scapula and forms the elbow joint with the lower arm bones. The radius and ulna are the two bones of the forearm.

The ulna is on the medial side of the forearm and forms a hinge joint with the humerus at the elbow. The radius allows the forearm and hand to turn over at the wrist joint. The lower arm bones form the wrist joint with the carpals, a group of eight small bones that give added flexibility to the wrist. The carpals are connected to the five metacarpals that form the bones of the hand and connect to each of the fingers. Each finger has three bones known as phalanges, except for the thumb, which only has two phalanges. Pelvic Girdle and Lower Limb Formed by the left and right hip bones, the pelvic girdle connects the lower limb (leg) bones to the axial skeleton. The femur is the largest bone in the body and the only bone of the thigh (femoral) region. The femur forms the ball and socket hip joint with the hip bone and forms the knee joint with the tibia and patella. Commonly called the kneecap, the patella is special because it is one of the few bones that are not present at birth. The patella forms in early childhood to support the knee for walking and crawling.

The tibia and fibula are the bones of the lower leg. The tibia is much larger than the fibula and bears almost all of the body’s weight. The fibula is mainly a muscle attachment point and is used to help maintain balance. The tibia and fibula form the ankle joint with the talus, one of the seven tarsal bones in the foot . The tarsals are a group of seven small bones that form the posterior end of the foot and heel. The tarsals form joints with the five long metatarsals of the foot. Then each of the metatarsals forms a joint with one of the set of phalanges in the toes. Each toe has three phalanges, except for the big toe, which only has two phalanges. Microscopic Structure of Bones The skeleton makes up about 30-40% of an adult’s body mass. The skeleton’s mass is made up of nonliving bone matrix and many tiny bone cells. Roughly half of the bone matrix’s mass is water , while the other half is collagen protein and solid crystals of calcium carbonate and calcium phosphate. Living bone cells are found on the edges of bones and in small cavities inside of the bone matrix.

Although these cells make up very little of the total bone mass, they have several very important roles in the functions of the skeletal system. The bone cells allow bones to: Grow and develop Be repaired following an injury or daily wear Be broken down to release their stored minerals Types of Bones All of the bones of the body can be broken down into five types: long, short, flat, irregular, and sesamoid. Long. Long bones are longer than they are wide and are the major bones of the limbs. Long bones grow more than the other classes of bone throughout childhood and so are responsible for the bulk of our height as adults. A hollow medullary cavity is found in the center of long bones and serves as a storage area for bone marrow. Examples of long bones include the femur, tibia, fibula, metatarsals, and phalanges. Short. Short bones are about as long as they are wide and are often cubed or round in shape. The carpal bones of the wrist and the tarsal bones of the foot are examples of short bones. Flat. Flat bones vary greatly in size and shape, but have the common feature of being very thin in one direction.

Because they are thin, flat bones do not have a medullary cavity like the long bones. The frontal, parietal, and occipital bones of the cranium—along with the ribs and hip bones—are all examples of flat bones. Irregular. Irregular bones have a shape that does not fit the pattern of the long, short, or flat bones. The vertebrae, sacrum, and coccyx of the spine—as well as the sphenoid, ethmoid, and zygomatic bones of the skull—are all irregular bones. Sesamoid. The sesamoid bones are formed after birth inside of tendons that run across joints. Sesamoid bones grow to protect the tendon from stresses and strains at the joint and can help to give a mechanical advantage to muscles pulling on the tendon. The patella and the pisiform bone of the carpals are the only sesamoid bones that are counted as part of the 206 bones of the body. Other sesamoid bones can form in the joints of the hands and feet, but are not present in all people. Parts of Bones The long bones of the body contain many distinct regions due to the way in which they develop.

At birth, each long bone is made of three individual bones separated by hyaline cartilage. Each end bone is called an epiphysis (epi = on; physis = to grow) while the middle bone is called a diaphysis (dia = passing through). The epiphyses and diaphysis grow towards one another and eventually fuse into one bone. The region of growth and eventual fusion in between the epiphysis and diaphysis is called the metaphysis (meta = after). Once the long bone parts have fused together, the only hyaline cartilage left in the bone is found as articular cartilage on the ends of the bone that form joints with other bones. The articular cartilage acts as a shock absorber and gliding surface between the bones to facilitate movement at the joint. Looking at a bone in cross section, there are several distinct layered regions that make up a bone. The outside of a bone is covered in a thin layer of dense irregular connective tissue called the periosteum. The periosteum contains many strong collagen fibers that are used to firmly anchor tendons and muscles to the bone for movement.

Stem cells and osteoblast cells in the periosteum are involved in the growth and repair of the outside of the bone due to stress and injury. Blood vessels present in the periosteum provide energy to the cells on the surface of the bone and penetrate into the bone itself to nourish the cells inside of the bone. The periosteum also contains nervous tissue and many nerve endings to give bone its sensitivity to pain when injured. Deep to the periosteum is the compact bone that makes up the hard, mineralized portion of the bone. Compact bone is made of a matrix of hard mineral salts reinforced with tough collagen fibers. Many tiny cells called osteocytes live in small spaces in the matrix and help to maintain the strength and integrity of the compact bone. Deep to the compact bone layer is a region of spongy bone where the bone tissue grows in thin columns called trabeculae with spaces for red bone marrow in between. The trabeculae grow in a specific pattern to resist outside stresses with the least amount of mass possible, keeping bones light but strong. Long bones have a spongy bone on their ends but have a hollow medullary cavity in the middle of the diaphysis.

The medullary cavity contains red bone marrow during childhood, eventually turning into yellow bone marrow after puberty. Articulations An articulation, or joint, is a point of contact between bones, between a bone and cartilage, or between a bone and a tooth. Synovial joints are the most common type of articulation and feature a small gap between the bones. This gap allows a free range of motion and space for synovial fluid to lubricate the joint. Fibrous joints exist where bones are very tightly joined and offer little to no movement between the bones. Fibrous joints also hold teeth in their bony sockets. Finally, cartilaginous joints are formed where bone meets cartilage or where there is a layer of cartilage between two bones. These joints provide a small amount of flexibility in the joint due to the gel-like consistency of cartilage. Skeletal System Physiology Support and Protection The skeletal system’s primary function is to form a solid framework that supports and protects the body's organs and anchors the skeletal muscles. The bones of the axial skeleton act as a hard shell to protect the internal organs—such as the brain and the heart —from damage caused by external forces.

The bones of the appendicular skeleton provide support and flexibility at the joints and anchor the muscles that move the limbs. Movement The bones of the skeletal system act as attachment points for the skeletal muscles of the body. Almost every skeletal muscle works by pulling two or more bones either closer together or further apart. Joints act as pivot points for the movement of the bones. The regions of each bone where muscles attach to the bone grow larger and stronger to support the additional force of the muscle. In addition, the overall mass and thickness of a bone increase when it is under a lot of stress from lifting weights or supporting body weight. Hematopoiesis Red bone marrow produces red and white blood cells in a process known as hematopoiesis. Red bone marrow is found in the hollow space inside of bones known as the medullary cavity . Children tend to have more red bone marrow compared to their body size than adults do, due to their body’s constant growth and development. The amount of red bone marrow drops off at the end of puberty, replaced by yellow bone marrow. Storage The skeletal system stores many different types of essential substances to facilitate growth and repair of the body.

The skeletal system’s cell matrix acts as our calcium bank by storing and releasing calcium ions into the blood as needed. Proper levels of calcium ions in the blood are essential to the proper function of the nervous and muscular systems. Bone cells also release osteocalcin, a hormone that helps regulate blood sugar and fat deposition. The yellow bone marrow inside of our hollow long bones is used to store energy in the form of lipids. Finally, red bone marrow stores some iron in the form of the molecule ferritin and uses this iron to form hemoglobin in red blood cells. Growth and Development The skeleton begins to form early in fetal development as a flexible skeleton made of hyaline cartilage and dense irregular fibrous connective tissue. These tissues act as a soft, growing framework and placeholder for the bony skeleton that will replace them. As development progresses, blood vessels begin to grow into the soft fetal skeleton, bringing stem cells and nutrients for bone growth. Osseous tissue slowly replaces the cartilage and fibrous tissue in a process called calcification. The calcified areas spread out from their blood vessels replacing the old tissues until they reach the border of another bony area.

At birth, the skeleton of a newborn has more than 300 bones; as a person ages, these bones grow together and fuse into larger bones, leaving adults with only 206 bones. Flat bones follow the process of intramembranous ossification where the young bones grow from a primary ossification center in fibrous membranes and leave a small region of fibrous tissue in between each other. In the skull these soft spots are known as fontanels, and give the skull flexibility and room for the bones to grow. Bone slowly replaces the fontanels until the individual bones of the skull fuse together to form a rigid adult skull. Long bones follow the process of endochondral ossification where the diaphysis grows inside of cartilage from a primary ossification center until it forms most of the bone. The epiphyses then grow from secondary ossification centers on the ends of the bone. A small band of hyaline cartilage remains in between the bones as a growth plate. As we grow through childhood, the growth plates grow under the influence of growth and sex hormones, slowly separating the bones. At the same time the bones grow larger by growing back into the growth plates.

This process continues until the end of puberty, when the growth plate stops growing and the bones fuse permanently into a single bone. The vast difference in height and limb length between birth and adulthood are mainly the result of endochondral ossification in the long bones. Prepared by Tim Taylor, Anatomy and Physiology Instructor Your Bones Your Bones Tus huesos Think back to last Halloween for a minute. Wherever you looked, there were vampires, ghosts, or bony skeletons grinning back at you. Vampires and ghosts don't really exist, but skeletons sure do! Every single person has a skeleton made up of many bones. These bones give your body structure, let you move in many ways, protect your internal organs, and more. It's time to look at all your bones — the adult human body has 206 of them! What Are Bones Made Of? If you've ever seen a real skeleton or fossil in a museum, you might think that all bones are dead. Although bones in museums are dry, hard, or crumbly, the bones in your body are different. The bones that make up your skeleton are all very much alive, growing and changing all the time like other parts of your body.

Almost every bone in your body is made of the same materials: The outer surface of bone is called the periosteum (say: pare-ee-OSS-tee-um). It's a thin, dense membrane that contains nerves and blood vessels that nourish the bone. The next layer is made up of compact bone. This part is smooth and very hard. It's the part you see when you look at a skeleton. Within the compact bone are many layers of cancellous (say: KAN-sell-us) bone, which looks a bit like a sponge. Cancellous bone is not quite as hard as compact bone, but it is still very strong. In many bones, the cancellous bone protects the innermost part of the bone, the bone marrow (say: MAIR-oh). Bone marrow is sort of like a thick jelly, and its job is to make blood cells. continue How Bones Grow When you were a baby, you had tiny hands, tiny feet, and tiny everything! Slowly, as you grew older, everything became a bit bigger, including your bones. A baby's body has about 300 bones at birth. These eventually fuse (grow together) to form the 206 bones that adults have.

Some of a baby's bones are made entirely of a special material called cartilage (say: KAR-tel-ij). Other bones in a baby are partly made of cartilage. This cartilage is soft and flexible. During childhood, as you are growing, the cartilage grows and is slowly replaced by bone, with help from calcium. By the time you are about 25, this process will be complete. After this happens, there can be no more growth — the bones are as big as they will ever be. All of these bones make up a skeleton that is both very strong and very light. Your Spine Your spine is one part of the skeleton that's easy to check out: Reach around to the center of your back and you'll feel its bumps under your fingers. The spine lets you twist and bend, and it holds your body upright. It also protects the spinal cord, a large bundle of nerves that sends information from your brain to the rest of your body. The spine is special because it isn't made of one or even two bones: It's made of 26 bones in all! These bones are called vertebrae (say: VER-tuh-bray) and each one is shaped like a ring.

There are different types of vertebrae in the spine and each does a different kind of job: The first seven vertebrae at the top are called the cervical (say: SIR-vih-kul) vertebrae. These bones are in the back of your neck, just below your brain, and they support your head and neck. Your head is pretty heavy, so it's lucky to have help from the cervical vertebrae! Below the cervical vertebrae are the thoracic (say: thuh-RAS-ik) vertebrae, and there are 12 in all. These guys anchor your ribs in place. Below the thoracic vertebrae are five lumbar (say: LUM-bar) vertebrae. Beneath the lumbar vertebrae is the sacrum (say: SAY-krum), which is made up of five vertebrae that are fused together to form one single bone. Finally, all the way at the bottom of the spine is the coccyx (say: COK-siks), which is one bone made of four fused vertebrae. The bottom sections of the spine are important when it comes to bearing weight and giving you a good center of gravity.

So when you pick up a heavy backpack, the lumbar vertebrae, sacrum, and coccyx give you the power. When you dance, skip, and even walk, these parts help keep you balanced. In between each vertebra (the name for just one of the vertebrae) are small disks made of cartilage. These disks keep the vertebrae from rubbing against one another, and they also act as your spine's natural shock absorbers. When you jump in the air, or twist while slamming a dunk, the disks give your vertebrae the cushioning they need. previous continue Your Ribs Your heart, lungs, and liver are all very important, and luckily you've got ribs to keep them safe. Ribs act like a cage of bones around your chest. It's easy to feel the bottom of this cage by running your fingers along the sides and front of your body, a few inches below your heart . If you breathe in deeply, you can easily feel your ribs right in the front of your body, too. Some thin kids can even see a few of their ribs right through their skin. Your ribs come in pairs, and the left and right sides of each pair are exactly the same.

Most people have 12 pairs of ribs, but some people are born with one or more extra ribs, and some people might have one pair less. All 12 pairs of ribs attach in the back to the spine, where they are held in place by the thoracic vertebrae. The first seven pairs of ribs attach in the front to the sternum (say: STUR-num), a strong bone in the center of your chest that holds those ribs in place. The remaining sets of ribs don't attach to the sternum directly. The next three pairs are held on with cartilage to the ribs above them. The very last two sets of ribs are called floating ribs because they aren't connected to the sternum or the ribs above them. But don't worry, these ribs can't ever float away. Like the rest of the ribs, they are securely attached to the spine in the back. Your Skull Your skull protects the most important part of all, the brain. You can feel your skull by pushing on your head, especially in the back a few inches above your neck. The skull is actually made up of different bones. Some of these bones protect your brain, whereas others make up the structure of your face.

If you touch beneath your eyes, you can feel the ridge of the bone that forms the hole where your eye sits. And although you can't see it, the smallest bone in your whole body is in your head, too. The stirrup bone behind your eardrum is only .1 to .13 inches (2.5 to 3.3 millimeters) long! Want to know something else? Your lower jawbone is the only bone in your head you can move. It opens and closes to let you talk and chew food. Your skull is pretty cool, but it's changed since you were a baby. All babies are born with spaces between the bones in their skulls. This allows the bones to move, close up, and even overlap as the baby goes through the birth canal. As the baby grows, the space between the bones slowly closes up and disappears, and special joints called sutures (say: SOO-churs) connect the bones. previous continue Your Hands As you sit and type at the keyboard, while you swing on a swing, even when you pick up your lunch, you're using the bones in your fingers, hand, wrist, and arm.

Each arm is attached to a shoulder blade or scapula (say: SKA-pyuh-luh), a large triangular bone on the upper back corner of each side of the ribcage. The arm is made up of three bones: the humerus (say: HYOO-muh-rus), which is above your elbow, and the radius (say: RAY-dee-us) and ulna (say: UL-nuh), which are below the elbow. Each of these bones is wider at the ends and skinnier in the middle, to help give it strength where it meets another bone. At the end of the radius and ulna are eight smaller bones that make up your wrist. Although these bones are small, they can really move! Twist your wrist around or wave and you'll see how the wrist can move. The center part of your hand is made up of five separate bones. Each finger on your hand has three bones, except for your thumb, which has two. So between your wrists, hands, and all your fingers, you've got a grand total of 54 bones — all ready to help you grasp things, write your name, pick up the phone, or throw a softball!

Your Legs Sure, your arm, wrist, hand, and finger bones are great for picking up the phone, but how are you supposed to run to answer it? Well, with the bones of the legs and feet! Your legs are attached to a circular group of bones called your pelvis. The pelvis is a bowl-shaped structure that supports the spine. It is made up of the two large hip bones in front, and behind are the sacrum and the coccyx. The pelvis acts as a tough ring of protection around parts of the digestive system, parts of the urinary system, and parts of the reproductive system. Your leg bones are very large and strong to help support the weight of your body. The bone that goes from your pelvis to your knee is called the femur (say: FEE-mur), and it's the longest bone in your body. At the knee, there's a triangular-shaped bone called the patella (say: puh-TEL-luh), or kneecap, that protects the knee joint. Below the knee are two other leg bones: the tibia (say: TIH-bee-uh) and the fibula (say: FIH-byuh-luh).

Just like the three bones in the arm, the three bones in the leg are wider at the ends than in the middle to give them strength. The ankle is a bit different from the wrist; it is where the lower leg bones connect to a large bone in the foot called the talus (say: TAL-iss). Next to the talus are six other bones. But the main part of the foot is similar to the hand, with five bones. Each toe has three tiny bones, except for your big toe, which has just two. This brings the bone total in both feet and ankles to 52! Most people don't use their toes and feet for grabbing stuff or writing, but they do use them for two very important things: standing and walking. Without all the bones of the foot working together, it would be impossible to balance properly. The bones in the feet are arranged so the foot is almost flat and a bit wide, to help you stay upright. So the next time you're walking, be sure to look down and thank those toes! Your Joints The place where two bones meet is called a joint. Some joints move and others don't. Fixed joints are fixed in place and don't move at all.

Your skull has some of these joints (called sutures, remember?), which close up the bones of the skull in a young person's head. One of these joints is called the parieto-temporal (say: par-EYE-ih-toh TEM-puh-rul) suture — it's the one that runs along the side of the skull. Moving joints are the ones that let you ride your bike, eat cereal, and play a video game — the ones that allow you to twist, bend, and move different parts of your body. Some moving joints, like the ones in your spine, move only a little. Other joints move a lot. One of the main types of moving joints is called a hinge joint. Your elbows and knees each have hinge joints, which let you bend and then straighten your arms and legs. These joints are like the hinges on a door. Just as most doors can only open one way, you can only bend your arms and legs in one direction. You also have many smaller hinge joints in your fingers and toes. Another important type of moving joint is the ball and socket joint. You can find these joints at your shoulders and hips.

They are made up of the round end of one bone fitting into a small cup-like area of another bone. Ball and socket joints allow for lots of movement in every direction. Make sure you've got lots of room, and try swinging your arms all over the place. Have you ever seen someone put oil on a hinge to make it work easier or stop squeaking? Well, your joints come with their own special fluid called synovial fluid (say: SIH-no-vee-ul) that helps them move freely. Bones are held together at the joints by ligaments (say: LIH-guh-mints), which are like very strong rubber bands. Taking Care of Bones Your bones help you out every day so make sure you take care of them. Here are some tips: Protect those skull bones (and your brain inside!) by wearing a helmet for bike riding and other sports. When you use a skateboard, in-line skates, or a scooter, be sure to add wrist supports and elbow and knee pads. Your bones in these places will thank you if you have a fall! If you play sports like football, soccer, lacrosse, or ice hockey, always wear all the right equipment. And never play on a trampoline.

Many kids end up with broken bones from jumping on them. Broken bones can eventually heal, but it takes a long time and isn't much fun while you wait. Strengthen your skeleton by drinking milk and eating other dairy products (like low-fat cheese or frozen yogurt). They all contain calcium, which helps bones harden and become strong. Be active! Another way to strengthen your bones is through exercise like running, jumping, dancing, and playing sports. Take these steps to be good to your bones, and they will treat you right!

Question: Which bone in the human body is at the front but sounds like its at the back?
Answer: STERNUM